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Bob McDavitt's ideas for sailing weather around the South pacific

26 June 2010

BOBGRAM7 issued 27 June 2010

WEATHERGRAM
YOTREPS
Issued 27 June 2010
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Galapagos to Marquesas: trade winds are now strong enough so that boats
faster than 6kt may sail this route directly, hugging 4S as long as
possible. Slower boats and those looking for an alternative route
should get to may between 1 deg and 2deg North of the equator for a free
ride on the equatorial west-going current, especially from 095 to 125W.
No weather to mention, but note that in the Northern hemisphere near
the Americas Cyclone CELIA is still going and Cyclone Darby is about as
well.

TROPICS
South Pacific Convergence zone SPCZ is active along 8 to 10S across the
Solomons area to Tuvalu and Tokelau, and then active again across French
Polynesia. There are occasional bursts of activity occurring about
northern Vanuatu and northern Fiji, but nothing organised.
Due to High pressures that are now blocked over southern Australia there
is a zone of enhanced trade winds in the Western Coral Sea and around
the north end of Queensland. This should ease after 30 June.

SUBTROPICAL RIDGE: STR
Large HIGH over 1030hpa to south of southern cooks / at 160W / is moving
steadily east along 40S. Its associated squash zone of enhanced trade
winds should affect southern parts of French Polynesia this week.

Small HIGH around 1020hPa is moving along 25S and should get from south
of New Caledonia today to south of Niue tomorrow/28 June. Another small
High around 1020hPa is likely between 25 and 30S to south of New
Caledonia on 29 to 30 June , and should then fade.

And the next HIGH is likely to bring a period of light winds across the
central Tasman Sea and NZ building to 1030+ from 5 to 7 July next
week...- a window that motor vessels may use to help cross the ditch as
comfortably as possible (and sailing vessels may wish to avoid).

TASMAN/NZ
This week in the local region we have just two small cells in the STR
around 25/30S, so the whole Taman Sea/NZ /east of the dateline area is
open to having a broad trough.
There should be one front/trough crossing NZ on Monday/Tuesday 28/29
June, followed by a disturbed W/SW flow for the remainder of the week.
After this SW wind change gets to Northland on wed 30 June, there should
be a reasonable window from sailing away from Northland. BUT note that
the frontal trough may deepen into a small low when it is north of NZ
and as it crosses near the Kermadecs on 2-3-4 June-so if heading for
Tonga ensure you go clockwise around it.

These conditions will make sailing TO New Zealand challenging this week.


The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

20 June 2010

BOBGRAM7 ISSUED 20 June 2010

WEATHERGRAM
YOTREPS
Issued 20 June 2010
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Galapagos to Marquesas: trade winds are now strong enough so that
anyone sailing this route may as well go direct, rather than chase the
extra help of the west going current that is around 30 miles north of
the equator. Head for 4S and then hug that latitude to 120W - that will
avoid a counter just west of Galapagos and prolong time in a west going
current near 4S. No weather to mention, but note that early this week
there are TWO cyclones in the NE Pacific, BLAS and CELIA.


TROPICS
South Pacific Convergence zone SPCZ is building in activity. It spent
last week over Coral Sea/Vanuatu, Fiji, and Samoa, then weaker between
Northern and Southern cooks, then strong again over French Polynesia FP.
It should reform slightly further north over Tuvalu and Tokelau this
week, and some of the convection in the SPCZ is likely to be drawn to
the SE by passing mid-latitude troughs - so look for troughs turning to
lows in the subtropics at around 20 to 30S from Sat 26 to Sun 27 June:
one near 180 and another near 120W/ SE of FP.


SUBTROPICAL RIDGE: STR
The squash zone and heavy swells over FP on Sun 20 June are related to a
HIGH at 40S, to Southeast of FP. As this wanders off to the east it
will take its squash zone with it.

HIGH crossing Tasmania on Tue 22 June should go east and get across
South Island on wed 24 June and then should slide northeastwards and
reach 30S to south of FP by Monday 28 June. cross the Tasman Sea on
Tue/Wed 15/16 June, and North Island on Wed 16/Thu 17 June, and then
slide off to the NE and get to move smartly along 40S to be south of
French Polynesia by Mon 21 June. Its migration to the east will be
accompanied by a squash zone in the trade winds on its northern side,
mainly along 20S. There will be a squash zone with this High, but
mainly on its NW shoulder.

Next HIGH is likely o be delayed getting across Australia and, at this
stage, is expected to reach Tasmania around 1 July - so next week, with
no Subtropical ridge around NZ, the stage is set for a large LOW over or
east of NZ.

TASMAN/NZ
Disturbed W/SW flow over NZ is good for those wishing to sail off north
today/Monday/Tuesday 21/22 June, but on wed 23 June an SE change is
likely to reach northland. This will be counter productive to sailing
and, combined with the movement of the High to the South Island is
likely to result in the formation of a LOW that may loop and linger
around Northland for the remainder of the week.

The next good sailing window from Northland will then be after this low
has moved sufficiently off to the east - probably Mon or Tue 28/29 June.
And that low is likely to deepen and expand in area as it goes east,
because of the weakness of the STR. If you are interested in this
window, then be aware that it may be bothered by lows and troughs
lingering around 30S, so re-check closer to the time. Also, around 1
July, between the expanding low to the east and the incoming High in the
South Tasman Sea, there should be a polar outbreak with cold
southerlies over NZ.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

13 June 2010

BOBGRAM7 issued 13 June 2010

WEATHERGRAM
YOTREPS
Issued 13 June 2010
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Galapagos to Marquesas: Apparently some are still "puddle jumping"---
There is a good west going current to be found at around 30 miles north
of the equator . Take this to around 123 W and then head direct for
Marquesas. Weather is looking settled this week.

TROPICS
South Pacific Convergence zone SPCZ is active mainly in the Eastern
Coral Sea, with scattered areas along 10S near Tuvalu to Tokelau/Samoa.
This week the convection to north of New Caledonia is expected to drift
east, and form a trough to east of New Caledonia. Around Wed 16 June a
low should start to form in this trough and then deepen and travel SE
to east of North island and South of Kermadecs by Sat 19 June. There
will be a squash zone between this LOW and a HIGH over NZ on 16 -17
June.

Anyone taking the opportunity to sail north from NZ when the SW winds
start easing on Mon 14 June and heading for Fiji or Tonga should ensure
they go clock-wise around this low and its squash zone.

SUBTROPICAL RIDGE: STR
HIGH should cross the Tasman Sea on Tue/Wed 15/16 June, and North Island
on Wed 16/Thu 17 June, and then move smartly along 40S to be south of
French Polynesia by Mon 21 June. Its migration to the east will be
accompanied by a squash zone in the trade winds on its northern side ,
mainly along 20S.
Good to see this pattern return, as it is the regular winter pattern.
That LOW mentioned as forming in the tropics will probably wait for this
High to get out of the way first.

Next HIGH may be delayed and knocked to 45S and move into South Tasman
Sea south of Tasmania on 22 June.

TASMAN/NZ
The arrival of the Wed/Thu High coincides with the start of the National
fieldays which I will be attending this week. Fog and frost will be on
the menu.

A front should cross NZ on Fri 18/sat 19 June following this High, but
this is expected to fade as it comes, so will not bring a SW change to
Northland.

A Low is forecast to form off the Queensland coast on Fri 18 June, and
this should move SE across the North Tasman Sea and then across
Northland around Tue 22 June Tuesday. This LOW may or may not be
able to time its arrival along with the outbreak of cold southerlies
that will be shovelled onto the South Island by the next HIGH. So NZ
may be in for another cold S/SE outbreak early next week.

This means that the NEXT SW wind change in Northland for good sailing
to the North has been delayed to maybe the end of next week /maybe the
end of the month. Tomorrows "window" closes quickly too.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

06 June 2010

BOBGRAM7 issued 6 June 2010

WEATHERGRAM
YOTREPS
Issued 6 June 2010
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Whoops - I see that last week's email heading was dated 30 July 2010
instead of 30 May 2010.

TROPICS
South Pacific Convergence zone SPCZ is still gradually reforming along
10S, from Papua New Guinea to east of Solomons to Tuvalu/Tokelau and
extending to Samoa at times, also extending south between Vanuatu and
Fiji at times. During this week it seems that there will be more
convective activity building in the Coral Sea, and this may culminate in
a trough/low forming west of New Caledonia by around 19 June (end of
NEXT week).

There has also been more rain clouds than normal in a zone from just
south of New Caledonia onto northern NZ . This zone isn't part of SPCZ,
rather it is a combination of the northern end of mid-latitude frontal
systems + jetstream intensification + a large upper low stuck in the
western Tasman Sea and developing a series of surface lows to throw at
NZ, like a bowler player cricket. This "low index" pattern started in
mid May just in time to harass this years Auckland to Fiji race. It
seems to me that the pattern is about to change this week to the more
familiar "Roaring 40s" pattern.


SUBTROPICAL RIDGE: STR
The STR has been absent over NZ since mid May. There have been thin
ridges squeezing along 20 to 30S - north of the normal latitude, making
for lighter than normal trade winds between Fiji and French Polynesia.
This is about to change. The next STR is over central New South Wales
today (sun 6 June) and should wander east between 30 and 40S, crossing
NZ on Wednesday 9 June, and then settle between 25 and 35 South as it
wanders east of NZ.

The following STR should be a large HIGH in the Aussie Bight on Fri 11
June. As it crosses Bass Strait on the weekend of 12 /13 June it should
shovel polar air onto southern NZ producing a solid dump of snow there.
Models indicate this High will not be able to get into the Tasman sea at
40S , so it should go N across eastern OZ on 14/15/16 June and then
move east along 30 to 35S - across northern Tasman sea and north of NZ
on 17/18/19 June--- with enhanced easterlies on its northern side and
enhanced westerlies on its southern side --- the "Roaring 40s" will be
back.


TASMAN/NZ
Low crossing central NZ on Monday should go southeast across Chathams on
Tuesday. There is a sting in its tail, as one more squally trough with
W to SW winds is expected to be crossing northern NZ on Tuesday as well.


Light winds with a ridge on Wednesday.

Trough/front is expected to move from Tasman Sea across NZ on Thursday
and early Friday. The SW winds following this trough should reach
Northland Friday morning, producing a reasonable looking window for
sailing northwards.

Additionally, there is a brief window late Tuesday and early Wednesday,
with the minor impediment of some pre-frontal northerly winds on
Thursday and - note well- following the Friday front - there is
expected to be a period of heavy westerly swell over the seas north of
Northland.

I'm gearing up now to attend to MetService display at Fieldays from Tue
15 to Sat 19 June. There should be a briefer weathergrams issued Sun 13
June, then I'll mostly be unavailable until 19 June,


The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

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